Carbon films, including carbon nanotube (CNT) materials, are being developed for cold cathode applications. These applications include field emission displays, x-ray tubes, microwave devices, CRTs, satellite thrusters, or any applications requiring a source of electrons. There are many types of carbon films that are being considered. The emission mechanism believed to be responsible for the emission of electrons from these carbon films is the Fowler-Nordheim theory; this is especially true for the carbon films that are conducting. Included in this emission mechanism is an electrical barrier at the surface of the conductor that prevents electrons from exiting the metal. However, if a strong field is applied, this barrier is lowered or made thin such that electrons can now “tunnel” through the barrier to create a finite emission current. The height of this barrier is partially determined by the work function at the particular surface of the material. The work function is dependent on the material, which surface of the material an attempt to extract electrons is being made, whether or not there are impurities on this surface and how the surface is terminated. What is important is that the lower the work function, the lower the barrier becomes and the easier it is to extract electrons from the carbon film. If a means or treatment is developed that lowers the value of the work function, then it becomes easier to extract electrons from the film; easier in the sense that lower extraction fields are required and higher currents can be obtained from treated films as opposed to untreated films operated at the same extraction field.
In analyzing field emission data, there are four unknowns in the Fowler-Nordheim (F-N) equation. These are n, α, β, and Φ with n the number of emission sites (e.g., tips), α the emission area per site, β the field enhancement factor and Φ the work function. The F-N equation is given by:I=αAexpBwithA=1.54 10−6 E2/ΦB=−6.87 107 Φ1.5 v/Ev=0.95−y2andy=3.79 10−4 E0.5/Φ
The field at an emission site is E=βE0 with E0=V/d where V is the extraction voltage and d the cathode-to-anode distance.
To see the effect that work function has on the field emission current, the graph in FIG. 1 shows how lowering the work function from 4.6 eV to 2.4 eV significantly lowers the threshold electric field and allows much higher current densities (orders of magnitude) at a given electrical field.
Single wall carbon nanotubes (SWNTs) and multiwall carbon nanotubes (MWNTs) can be used as carbon materials for field emission applications because they are tall and thin and have sharp features. These sharp features enhance the electric field at these points (large β), thus a larger field can be achieved with a given applied voltage. Being made of carbon, they are also conductive, mechanically very strong, and chemically robust. The work function of the SWNT material (4.8 eV) is slightly higher than graphite (4.6–4.7 eV), as disclosed in Suzuki et al., APL, Vol. 76, p. 4007, Jun. 26, 2000, which is hereby incorporated by reference herein.
What is needed is a means of optimizing the field emission properties of a carbon material by lowering the work function of this material. This would improve the emission characteristics of the carbon nanotube material, both SWNT and MWNT.